Jim, Could not resist, I am back,

Here is the relevant extract from the W124 "Mercedes Maintenance Manual". Procedure No: 20-310. This is also applicable to all other Mercedes models that use this type of Viscous Fan Clutch. If you like I can E-mail it to you.

--------------------------------------------------------------
"The Viscous fan coupling is a maintenance-free hydraulic coupling which operates steplessly dependent on temperature.

If the coolant temperature rises because of higher engine load or high outside temperatures, the air which flows through the radiator and impinges on the bimetal strip, becomes warmer. The bimetal strip alters its shape as it heats up and opens a valve at approx. 71C by means of a pin and thus also the passage of oil from the storage chamber into the working chamber, which causes the fan to cut in.

The coolant temperature during this switching operation is between approx. 90 - 95C
--------------------------------------------------------------

Jim, The above states that the Viscous Fan Clutch has been designed to engage at an air temperature of 71C, which is approximately equivalent to a coolant temperature of 90 - 95C. The Auxiliary fans on the other hand trigger at 100-105C. So by design, the Viscous Fan Clutch will engage BEFORE the auxiliary fans.

Phil

__________________
'95 E300 Diesel, 264,000 Miles. [Sold it]